There are places in the world where the salt concentration is already high enough to make any environmental impact from dumping more salt negligible.

That's not why I resurrected this old topic though.

I looked at a table for energy density, and noticed aluminium being very high on energy/weight and especially energy/volume compared to other substances. Hydrogen has great energy/weight ratio but poor energy/volume ratio. Would it be possible to use aluminium as fuel for an upper stage? Powdered aluminium mixed with some liquid fuel, perhaps? Or just liquid aluminium (melting point about 660 C)?